4.3. CMB Baryon Density - The Baryon Density
At A Few Hundred Thousand Years

As discussed in the first lecture, the early universe
is hot and dominated by relativistic particles
("radiation"). As the universe expands and cools,
nonrelativistic particles ("matter") come to dominate
after a few hundred thousand years, and any preexisting
density perturbations can begin to grow under the
influence of gravity. On length scales determined by
the density of baryons, oscillations ("sound waves")
in the baryon-photon fluid develop. At a redshift of
z ~ 1100 the electron-proton plasma combines
(``recombination) to form neutral hydrogen which is
transparent to the CMB photons. Free to travel throughout
the post-recombination universe, these CMB photons preserve
the record of the baryon-photon oscillations as small
temperature fluctuations in the CMB spectrum. Utilizing recent CMB
observations
(Lee et al. 2001;
Netterfield et al. 2002;
Halverson et al. 2002),
many groups have inferred the intermediate age universe baryon
density. The work of our group at OSU
(Kneller et al. 2001)
is consistent with more
detailed analyses and is the one I adopt for the purpose
of comparison with the SBBN result:
10 =
6.0 ± 0.6;
Bh2 = 0.022 ± 0.002.